1. Field of the Invention
The present invention is the result of collaboration with the "Laboratoire Image" of the "Ecole Nationale Superieure des Telecommunications". Its relates to a method for the computing and imaging of views of an object. It can be applied in a variety of fields where displays are required. In particular, it can be applied in medicine where, for diagnostic reasons, practitioners have to know the internal tissue structures of the bodies of patients being examined. The invention shall be described in this context without, however, any implication whatsoever that it is restricted to this context.
2. Description of the Prior Art
There are known methods in medicine for investigating the interior of the human body with a view to depicting its internal tissue structure. These known methods include X-ray tomodensitometry, nuclear magnetic resonance imaging, gammaradiography and echography. In all these techniques, and especially in the first one, there are known ways for depicting images of sections, along determined section planes, of the body examined. For a clearer understanding of this question, it might be recalled that an X-ray tomodensitometer has an X-ray generator which emits a flat (thin) beam of X-rays towards a multi-detector which is held facing this generator. A body to be examined is placed, with its longitudinal dimension arranged transversally with respect to the radiation plane, between this generator and this multi-detector. Then, the generator/multi-detector set is rotated around the body with a section of the body being subjected to successive doses of irradiation at several angles of incidence. At each irradiation, the detected signal is measured by the multi-detector. With the series of measurements corresponding to the several angles of incidence involved, an image of this section can be reconstructed according to known methods for reconstructing images. To obtain knowledge of the architecture of internal tissue structures, a third dimension must be explored. In practice, the body is shifted longitudinally with respect to the generator/multi-detector set, and the operation is started again for a section that neighbours the previous one. This procedure is continued until the collection of images, stacked on one another, is itself sufficient to enable the depiction of sections of the body in a variety of sectional directions. For the acquisition mode described can be used for the immediate depiction of cross-sections (roughly perpendicular to the patient's vertebral column). Then, image lines can be taken out of each image of the successive sections and can be recombined to create frontal sections (parallel to the patient's back) or again, sagittal sections (perpendicular to the patient's back). It is even possible to depict oblique sections through the judicious choice of image lines from each of the successive images.
It can be shown that this type of depiction is also possible with all the other three methods referred to. To conduct this type of operation efficiently, it is enough to have a set of data representing a physical phenonenon, namely radiological density, susceptibility to nuclear resonance, vascularization rate or the reflective power of ultrasonic waves, and for this data to be assigned to coordinates of regions in the space thus subjected to examination. All this data is defined by the term "digital volume". The word "volume" refers to the spatial distribution of regions or volume elements in the body under examination (these volume elements have three-dimensional coordinates x, y and z). The word "digital" refers to the physical data itself: in tomodensitometry, for example, it refers to the signal measured by the multi-detector. In the rest of this description, it shall be assumed that the spatial resolution of the digital volume is the same along all three directions in which the space concerned is explored. However, this is not a necessary condition for the invention. Furthermore, it is easy, in any case, to meet this condition by interpolating the physical data on neighbouring volume elements in the volume under examination.
The images thus shown have, however, the disadvantage of being sectional images. Sections impose constraints because their reading requires a priori interpretation by the reader. This means that the significance of a section is seen by the reader purely as a verification of his prior conclusions on this section. In addition to the fact that interpreting ability requires a great deal of experience, there are branches of science (especially nuclear magnetic resonance) where, since the investigative method used is a new one, the end image to be obtained is not always known, and it is even less easy to know how to interpret this image. Thus, there emerged the idea of showing views of objects rather than sections. It was thus felt that internal structures could be shown as they would be seen if the bodies examined had been dissected. The interpreting ability and competence needed to interpret a view are far less than would be the case for a section. This would result in a greatly improved understanding of the behaviour of the structures shown in the sections. The specialist is no longer required to reconstruct this behaviour in his mind.
In theory, the depiction of views of objects has already been studied. It comprises two main operations: firstly, a segmentation and secondly, the display itself. Taking a given and known digital volume, the segmentation uses the principle according to which an object (a structure within this digital volume) is determined by all the locations of the volume elements of this volume where the physical data measured has given characteristics. To simplify this point, in tomodensitometry, it might be broadly assumed that bones, for example, are differentiated from the other tissues by greater radiological density. It then becomes possible to distinguish all the volume elements, in the digital volume, for which the measured radiological density is above a certain threshold. More particularly, an object may be defined by a value of its physical data belonging to a range of values located on either side of a nominal value.
The images of the cross sections are generally prepared by computers and the contents of these images may be stored in the memories of these computers. These memories may be organized in memory sections representing each of the images, logically associated with one another to constitute a memory volume corresponding to the digital volume being examined. By means of a test, it is easy to determine those addresses of the memory cells, representating the digital volume elements (and hence representing a specific object in the volume examined in the body), in which the magnitude of the physical data stored comes within the range in question. The segmentation, namely the determination of the virtual image of the object to be depicted, is ended at this stage.
The display is then done according to techniques using the science of illumination. These techniques are applied to the segmented object for which the volume is known and for which the geometrical shape, i.e. the surface, can be known. Initially, the surface of this object is grasped, for example by means of many facets interconnecting the volume elements located on the boundary of the segmented object. Then these facets are assigned a luminosity level. This luminosity represents an illumination which they are capable of receiving and restoring according to their orientation. Finally, in the image of the view, surface elements of the image are devoted to the volume elements that are visible (from a given viewpoint), and these surface elements are assigned a luminosity level corresponding to that of the facets of the visible volume elements. As an indication, FIG. 1 gives a schematic view of a segmented volume of this type. By taking advantage of the 3-D (along three orthogonal spatial axes) matrix organization of the segmented object, the facets, such as 1, of the cubic volume elements of the segmented volume can be directly used as illuminated surfaces reflecting an illuminating light. This method which intuitively has great simplicity as regards the processing of the illumination signal because of the small number of illuminated surfaces that can be envisaged has, nonetheless, one disadvantage: it can only give views of an artificial contour, different from the real contour, because the perpendiculars to the facets are estimated in relation to the three cartesian axes describing this object (x, y, z). Other more efficient displaying techniques can be contemplated: they are all based on the same principle. The definition of the facets in these techniques is only more precise because the possibilities they give for orienting the facets are not limited and the views of the objects imaged are less crude. However, they also take longer to compute.
Various methods have been used to simplify the computation of images. For example, a method called "octree encoding" has been invented by D. J. Meagher and is disclosed in an article "Efficient Synthetic Image Generation of Arbitrary 3-D Objects" in IEEE Computer Society Conference on Pattern Recognition and Image Processing, June 1982. This method introduces a new presentation of segmentation. In it, each volume element is considered to be part of a group of eight volume elements adjacent to it. Each group is considered to be part of a set of eight groups adjacent to this group, and so on until the digital volume to be examined is completely defined. The elements in the groups, the groups in the sets, etc. are then assigned a binary value depending on whether or not they belong to the segmented object. This data structure has the advantage of providing for the simple creation of a number of algorithms, especially display algorithms, by using orders of priority with respect to the data. All the depiction methods referred to have one disadvantage: they assume that the contour presented is a true and sharply defined contour. However, real physical data, especially in medicine, is noise-infested. This noise, which may be due to measurement or processing, is sometimes loud enough to exclude certain volume elements from the segmented body when they actually belong to it. The result of this is that, if the place where this exclusion occurs is on the surface of the segmented object (the surface that it is desired to depict), the facets at this position get disoriented because of the hole created by the exclusion. And in the total image displayed, certain parts may appear clearly while the significance of other parts may be less clear. It is then impossible to process the image locally in order to improve or modify the characteristics of the segmentation or depiction of this image at the place where it is unclear. For any modification of the segmentation criteria (a modification or shift in the value of the range used) or of the display criteria (the definition of the facets) has repercussions on the totality of the displayed image. An improvement in one part of the image may then become a disturbance in another part. Such and such a part which has been clearly demarcated in a preceding image becomes imprecise in the new image after the criteria have been modified.
An object of the present invention is to remove these present disadvantages by assigning a parameter to each of the volume elements distinguished during the segmentation process. This parameter depends on the mode of exploration of the digital volume, and this parameter and mode of exploration are such that a different parameter is assigned to each of the different volume elements of the surface of the segmented object. In the present state of the art, the segmentation is done as follows: the memory cells of the memory are addressed, one after the other other, to a comparator which sorts them out according to a segmentation criterion. The mode in which the computer memory is addressed is then a mode that depends on the architectural technology of this computer. For example, the memory is addressed cell by cell in a memory line, line by line in a memory section, and section after section in the memory. In the invention, the memory addressing mode or (in what amounts to the same thing) the mode of exploration of the corresponding digital volume, is not laid down by the technology of the computer. On the contrary, it depends on a specific process. This specific process is related, in particular, to the location of the viewpoint from which the object is supposed to be displayed. Thus, each distinguished volume element is provided with a parameter, depending on a stage in the exploration mode at the end of which the volume element thus distinguished has been chosen. This parameter can then be used to modify the segmentation or depiction criteria locally.